The glaucomas are a group of potentially blinding optic neuropathies that are characterized by progressive pathological losses of the retinal ganglion cells (RGCs) and their axons that form the optic nerve. The causes are unknown and, therefore, the diagnosis and assessment of progression of disease depends on ophthalmic testing to identify and quantify clinical characteristics of glaucomatous neuropathy, such as the pattern of visual field defects and/or thinning of the retinal nerve fiber layer. To relate these clinical measurements to the basic pathology of glaucoma, a neuron doctrine for glaucoma has been proposed to correlate data from standard tests (standard automated perimetry and optical coherence tomography) to the loss of RGCs. The doctrine was derived through initial laboratory studies of experimental glaucoma in macaque monkeys and, then, modified and refined though patient-based clinical investigations. The final formulation of the doctrine produced concordance between subjective and objective measurements when the results were translated to their common parameter of RGCs, for both normal vision and defective vision from glaucoma. Thus, it was concluded that for individual patients, alterations in structure-function relationships usually should be in agreement for the degree and location of visual field defects caused by glaucoma.